Diode pumped solid-state laser is a rapidly growing field. Passively Q-switched micro-chip lasers are particularly interesting as they are able to provide short pulses (&lt;1 ns) with high peak power (kW) for moderate pump powers, from, for instance, a laser diode or the like, using a simple configuration.
A problem with passively Q-switched lasers is the large jitter in repetition rare. The jitter makes this type of laser impossible to use in a number of applications. In these applications the only possibility has been to use actively Q-switched lasers.
Different techniques for reducing the jitter in passively Q-switched lasers have been described and shown, for instance in "A stabilised mnicrochip laser" by M. Arvidsson et al, CLEO Europe 1996. Hamburg, Paper CFH2 and in "Characterization of Passively Q-switched Microchip Lasers for Laser Radar" by W. J. Manderville and K. M. Dinndorf, SPIE, vol. 2748, pp 358.
A passively Q-switched micro-chip laser for producing high-peak-power pulses of light of extremely short duration is also described in U.S. Pat. No. 5,394,413. A saturable absorber prevents the onset of lasing until the average inversion density within the cavity of the Q-switched laser reaches a predetermined value. The configuration of the laser is then such that, at onset of lasing the saturable absorber becomes transparent, i.e. it is said to be bleached, and a Q-switched output pulse having an extremely short length and high peak power is generated. The problem with this kind of Q-switched lasers is that the lasing times are dependent on its dimensions and not controllable in an exact way.
Actively Q-switched lasers, for instance described by Yariv A., "Optical Electronics in Modern Communications" Fifth Edition, pp 227 to 235, or by Wilson J. Et al, "Optoelectronics An Introduction", Second Edition, pp 226 to 230, demands a very high round trip loss from the active modulator. This loss is in the order of 100%, which requires a high voltage switching device.
However, actively Q-switched lasers, i.e. Q-switched lasers in which the control of the Q-switching is done directly at the Q-switch, for instance by changing the polarisation of the light, have other problems, such as high switching voltages, possibility of multiple pulsing due to piezoelectric effects and large laser cavities due to large size required regarding the active modulator. Actively Q-switched lasers also need fast high voltage switching power supplies to work.
There is a need for a Q-switched laser in a lot of applications having controllable pulsing, small size and not requiring the high voltage switching devices as nornally being required in actively Q-switched lasers.
A Q-switched mode-locked laser is disclosed in U.S. Pat. No, 4,019,156. This "dual modulation" laser is capable of producing transform-limited pulses having a controllable pulse duration and operates as a synchronous driven optical resonator. Intracavity spontaneous emission is gated symmetrically in time by a Pockels cell that is utilised to provide the first Q-switching and then 100% loss modulation in synchronisation with the pulse round trip time in the cavity. The function of the Pockels cell is thus to suppress the spontaneous emission generated by the active medium in order to let the laser be synchronised, to change the pulse length, and thereby synchonize the pulse length, and to provide order mode-locking. It does thus not participate in the very switching function. The function of the Pockels cell is here to provide a stabilisation. The time for each laser pulse could not be chosen at will but is determined by the design of the laser, i.e. the length of the cavity.